The invention relates to a picture display device having a vacuum envelope which is provided with a transparent face plate with a luminescent screen and with a rear wall, said display device comprising a plurality of juxtaposed sources for emitting electrons, a plurality of electron transport ducts cooperating with the sources and extending substantially parallel to the face plate for transporting electrons in the form of electron currents, and an active selection structure for withdrawing each electron current at predetermined locations from its transport duct and for directing said current towards desired pixels of the luminescent screen.
The display device described above is of the flat-panel type, as disclosed in EP-A-464 937. Display devices of the flat-panel type are devices having a transparent face plate and, arranged at a small distance therefrom, a rear plate, (which plates are interconnected by means of partitions) and in which the inner side of the face plate is provided with pixels in the form of a phosphor pattern, one side of which is provided with an electrically conductive coating (the combination generally being referred to as luminescent screen). If (video information-controlled) electrons impinge upon the luminescent screen, a visual image is formed which is visible via the front side of the face plate. The face plate may be flat or, if desired, curved (for example, spherical or cylindrical).
The display device described in EP-A-464 937 comprises a plurality of juxtaposed sources for emitting electrons, local electron transport ducts cooperating with the sources and each having walls of high-ohmic, electrically substantially insulating material having a secondary emission coefficient suitable for transporting emitted electrons in the form of electron currents and (colour) selection means comprising selectively energizable electrodes (selection electrodes) for withdrawing each electron current from its transport duct at predetermined extraction locations facing the luminescent screen, further means being provided for directing extracted electrons towards pixels of the luminescent screen for producing a picture composed of pixels.
The operation of the picture display device disclosed in EP-A-464 937 is based on the recognition that electron transport is possible when electrons impinge on an inner wail of an elongate evacuated cavity (referred to as "compartment") defined by walls of a high-ohmic, electrically substantially insulating material (for example, glass or synthetic material), if an electric field of sufficient power is generated in the longitudinal direction of the "compartment" (by applying a potential difference across the ends of the "compartment"). The impinging electrons then generate secondary electrons by wall interaction, which electrons are attracted to a further wall section and in their turn generate secondary electrons again by wall interaction. The circumstances (field strength E, electrical resistance of the walls, secondary emission coefficient .delta. of the walls) may be chosen to be such that a constant vacuum current will flow in the "compartment".
Starting from the above-mentioned principle, a flat-panel picture display device can be realised by providing each one of a plurality of juxtaposed "compartments", which constitute transport ducts, with a column of extraction apertures at a side which is to face a display screen. It will then be practical to arrange the extraction apertures of adjacent transport ducts along parallel lines extending transversely to the transport ducts. By adding selection electrodes arranged in rows to the arrangement of apertures, which selection electrodes are energizable by means of a first (positive) electric voltage (pulse) for withdrawing electron currents from the "compartments" via the apertures of a row, or which are energizable by means of a second (lower) electric voltage if no electrons are to be locally withdrawn from the "compartments", an addressing means is provided with which electrons withdrawn from the "compartments" can be directed (and accelerated) towards the screen for producing a picture composed of pixels by activating the pixels.
EP-A-464 937 describes a stepped active selection system comprising a preselection structure, a distribution duct structure and a fine-selection structure. A stepped selection system using a number of preselection extraction locations which is reduced with respect to the number of pixels, and an associated number of (fine) selection apertures which corresponds to the number of pixels provides advantages with respect to, for example, the extraction efficiency and/or the required number of electrical connections/drivers.
However, a drawback of the structure known from EPA-464 937 is that direct hits may occur. This means that there are electrons which are directly incident on non-selected fine-selection apertures. This is detrimental to colour purity and contrast.